At 9:00 a.m. a truck leaves the truck yard and travels west at a rate of 45 mi/hr. Two hours later, a second truck leaves along the same route, traveling at 55 mi/hr. When will the second truck catch up to the first?

The first truck is 2 x 45 = 90 miles ahead when the second truck starts.

Once they are both moving, the distance between them decreases at a rate of 10 mi/hr. It will therefore take 90/10 = nine hours. The time when the second truck catches up is 9 hours after 11 AM, or 8:00 PM.